Diversity method antenna devices have been conventionally known that have a plurality of antennas with different reception characteristics and directivities, and select therefrom an antenna in the best communication condition to receive radio waves (see JP 2002-237773, A and JP 2002-271125, A, for example).
In the diversity method antenna devices, the plurality of antennas are connected to a radio module through coaxial cables, and one or more of the antennas in good communication conditions are selected by the radio module. Therefore, good communication condition can be obtained even in the case where reception radio waves vary in arrival direction with surrounding environments or the case where reflected waves or diffracted waves of the radio waves are received.
Alternatively, antenna devices have been known that can obtain good communication condition by varying directions of an antenna (see JP 9-246824, A and JP 6-47171, A).
For example, as shown in FIG. 5, an antenna device described in JP 9-246824, A includes a telescopic antenna 9 slidably projecting from an end of a case 8. A recess 81 is defined at the end of the case 8, and an opening 82 is defined at a bottom of the recess 81. Inside the case 8, a cylindrical antenna holder 83 is attached facing to the opening 82. The antenna 9 has a first rod 911 having the largest diameter among a plurality of telescopic rod portions 91, the first rod 911 coupled to an antenna base 93 through a hinge mechanism 92. The antenna base 93 is slidably held by the antenna holder 83, and can rotate on the sliding direction.
In the antenna device, the antenna 9 can be rotated by the hinge mechanism 92 so as to be close to or apart from the case 8, while the antenna 9 can rotate on a central axis thereof to vary directions of a rotation axis of the hinge mechanism 92. Therefore, the antenna 9 can be directed in a direction of the highest reception sensitivity to thereby obtain good communication condition.
However, in the diversity method antenna devices, although an antenna in the best communication condition can be selected from the plurality of antennas, a direction of the antenna cannot be changed. Therefore, when a directivity of the antenna is out of an arrival direction of radio waves, the best reception condition cannot be obtained.
In the antenna device capable of varying directions of the antenna as shown in FIG. 5, although the antenna can be directed in a direction of the highest reception sensitivity, inherent reception characteristics of the antenna cannot be changed. Therefore, when the reception characteristics of the antenna are inappropriate for reception of intended radio waves, the best reception condition cannot be obtained.
Accordingly, an object of the present invention is to provide an antenna device capable of always realizing the best reception condition regardless of arrival directions of radio waves or inherent reception characteristics of each antenna.